1. Field of the Invention
The invention relates to a comparator.
2. Related Background Art
FIG. 3 shows an example of a circuit of a conventional comparator. V.sub.3 denotes a reference voltage source; I.sub.7 indicates a constant current source; Q.sub.21 to Q.sub.25 transistors; and R.sub.1 and R.sub.2 resistors.
A circuit which is constructed by the constant current source I.sub.7 and the transistors Q.sub.21 to Q.sub.24 is a simple comparator. An input terminal IN and the resistor R.sub.2 are connected to a base of the transistor Q.sub.21 and a base of the transistor Q.sub.25 through wires and connectors from another electronic circuit board, respectively.
The operation of the comparator of FIG. 3 will now be described hereinbelow.
In the case where the input terminal IN and the resistor R.sub.2 are respectively connected to the above transistors Q.sub.21 and Q.sub.25, the transistor Q.sub.25 is turned off and the circuit operates as an ordinary comparator.
When the terminal IN and the resistor R.sub.2 are not connected, an output of the comparator is an output in a state in which a high level signal has been applied to an input. In this state, if a problem logically occurs, the transistor Q.sub.25 connected by the resistor R.sub.1 is set into a saturation state by a power source V.sub.cc, thereby fixing a base potential of the transistor Q.sub.21 to a low level and satisfying the logic.
The conventional comparator, however, has the following technical problems to be solved.
(1) Since the wires or connectors must be added to know the coupling state, the costs rise. PA1 (2) Since the wires or connectors are needed to know the coupling state in addition to a purpose of the input of the comparator, it is impossible to know the coupling states of both of them. Then a disconnection to each wire or a damage of the connector occurs, the normal operation cannot be performed and there is a fear of malfunction.